Polyimide films have high heat resistance and high electrical insulating properties, and meet handling requirements with regard to rigidity, heat resistance, or electrical insulating properties. Polyimide films are therefore widely used in industrial fields such as electrical insulating films, heat insulating films, base films for flexible circuit substrates, and solar cell substrates.
A polyimide film is manufactured by casting a polyimide precursor solution including polyamic acid or the like onto a support and drying the product to form a self-supporting film, and heat-treating the self-supporting film.
The heat-treated polyimide film has residual stress, and a stress relaxation treatment (annealing treatment) is therefore performed to relax the residual stress and enhance dimensional stability to heat.
For example, Patent Document 1 discloses manufacturing a polyimide film having improved heat-shrinking properties by heating a polyimide film in an essentially tensionless state at 150° C. to 420° C. for 1 second to 60 minutes, then cooling the polyimide film to room temperature.
Patent Document 2 discloses manufacturing a low-shrinkage polyimide film by continuously heat-treating a polyimide film in a hot airstream while applying a tension of 1 to 10 kg/m thereto in the length direction of the film, and then performing a cooling treatment.
Patent Document 3 discloses manufacturing a low-shrinkage polyimide film by irradiating a polyimide film with far infrared rays and heat-treating the polyimide film in a short time while maintaining a tension of 1 kg/m to 10 kg/m thereon in the length direction of the polyimide film, then performing a cooling treatment.
The applicability of stress relaxation treatments is not limited to polyimide films; various types of films are subjected to stress relaxation treatments in order to relax residual stress and enhance dimensional stability to heat.